In order to study the role of dipeptidyl carboxypeptidase in enkephalin metabilism, a new enzyme assay method, using (DNS)(3H)-Gly-Gly-Phe-Leu as substrate was developed. The enkephalin inactivating dipeptidyl carboxypeptidase (enk inact. DPC) was inhibited markedly by acethiorphan but not by angiotensin converting enzyme inhibitor SQ 20881 or captopril. The regional distribution of enk inact. DPC in the brain parallels closely with that of enkephalins except in the region of cerebellum. Met5-enkephalin-Arg6-Phe7 was readily converted to met5-enkephalin also by a dipeptidyl carboxypeptidase. This converting activity was inhibited by the angiotensin converting enzyme inhibitor SQ 20881 but not by acethiorphan. With the selective inhibitors of the dipeptidyl carboxypeptidase, we are currently studying the physiological role of the dipeptidyl carboxypeptidase, in the metabolism of enkephalins and met5-enkephalin-Arg6-Phe7.